1. Field of the Invention
The present invention relates to a method and apparatus for cooling molded plastic articles after molding is finished. In particular, the present invention relates to method and apparatus for a post mold cooling (“PMC”) device having at least two opposed faces. The method and apparatus are particularly well suited for cooling injection molded thermoplastic polyester polymer materials such as polyethylene terephthalate (“PET”) preforms.
2. Related Art
A variety of post mold cooling methods are currently employed on injection molding machines (e.g., an Index machine platform) in various sequences to optimize the cooling of freshly molded plastic parts. Some parts (for example plastic preforms) are typically injection-molded using PET resin, and can have wall thicknesses varying from about 2.00 mm to greater than 4.00 mm, and consequently require extended cooling periods to solidify into substantially defect-free parts. Heavy walled parts (such as those made from a material that has a high resistance to thermal heat transfer, like plastic resin) can exhibit “reheating” phenomena that can produce defective parts after they have been ejected from the mold.
In the case of PET preforms some of these manufacturing defects are:                Crystallinity: The resin recrystallizes due to the elevated temperature of the core resin not cooling quickly enough. The white appearance of the crystals impairs the clarity of the final product and provides an area of potentially weakness in a resultant blown product.        Surface blemishes: The ejected performs, initially having solidified surfaces are reheated by the core material which causes the surface to soften and be easily marred. Sometimes this surface reheating can be severe enough to cause touching parts to weld together.        Geometric inaccuracies: Handling partly-cooled performs or attempting to further cool them in devices that do not maintain their geometric shape while their surfaces are reheated can cause the preform's round diameter to become oval shaped or the smooth surface to become wrinkled or non-linear.        
The above-noted problems could be alleviated somewhat by extending the cooling time of the injection molded performs in their mold. However, this will cause the injection molding cycle to be lengthy, typically 25 seconds or longer, wherein the majority of this time was used solely for cooling purposes. In an effort to improve the production efficiency of this process, several techniques are employed to perform a post mold cooling function, wherein partially cooled preforms are ejected from the injection mold after an initially cooled surface skin has formed to allow the part to be ejected without deformation. The partially cooled preforms are then handed off to a downstream device that continues to hold the preform while removing the remaining heat so that the preform can subsequently be handled without damage. Typically, the preform surface temperature needs to be lowered to about 70° C. to ensure safe handling.
The early ejection of partially cooled preforms released the injection molding equipment earlier in the molding cycle, thereby significantly improving the production efficiency of the equipment. Injection molding cycle times typically were halved from 25 seconds to about 12 seconds or less in some instances depending on the preform design being molded.
Some examples of post mold cooling technology are shown in U.S. Pat. Nos.: 4,729,732; Re. 33,237; 5,447,426; and 6,171,541.
Another approach to extending the cooling time for preforms is to utilize a turret molding machine in which more than one set of injection molding cores are employed. An example is the Index machine, shown in U.S. Pat. Nos.: 5,728,409; 5,830,404; 5,750,162; and 5,817,345, which disclose using a turret block having four faces and four core sets that are sequentially mated with one cavity set to hold the injection mold preforms. Preforms molded on this kind of equipment can be produced in molding cycle times of typically 10–13 seconds.
In Index machines with fewer core side tooling sets employed, additional post mold cooling devices are used to complete the preform cooling and maintain cycle time benefits. Examples of Index machines with post mold cooling devices are shown in U.S. Pat. Nos.: 6,059,557; 6,123,538; 6,143,225; and 6,113,834.
One technique for improving the rate of heat transfer from a cooling preform is to pressurize its interior volume while cooling it in a cavity. This method helps keep the preform's exterior surface in contact with the cooling cavity surface, and counters the shrinkage of the preform that tends to separate the two surfaces. This allows good heat transfer to be maintained. Examples of pressurized preform cooling are shown in U.S. Pat. Nos.: 4,950,152; and 5,679,306, and in EP 0 900 135.
U.S. Pat. No. 6,113,834 to Kozai discloses a post mold cooling device (PMC) that unloads preforms from an Index preform molding machine into cooling tubes. FIGS. 8–14 disclose mounting multiple sets of tubes on a single plate so that multiple molding sets of preforms can be cooled by the tubes during several injection molding cycles, thereby extending the cooling time of each preform during its stay in the PMC device. This patent also discloses blowing a cooling fluid from the PMC device onto the exposed gate area of the freshly molded preforms while they are still on the injection molding cores prior to their transfer to the PMC device. However, this post mold cooling technique is somewhat limited by the absence of a more efficient means to cool the interior of the preform. Also, there is no means for efficiently translating the turret to access the various sets of cooling tubes.
Therefore, there is a need for a post-mold cooling method and apparatus, which provides rapid, efficient cooling while further reducing the molding cycle time to further decrease the cost of producing molded plastic pieces.